The invention relates generally to a method and apparatus for making retreaded tires and, more particularly, to a method and apparatus in which the existing tread and material of a tire casing is removed to provide a tire casing having a desired circumference (or radius) onto which a new tread may be installed.
Retreaded tires provide an economical way to gain additional use from tire casings after the original tread or retread has become worn. According to a conventional method of retreading, sometimes referred to as cold process retreading, worn tire tread and other materials on a used tire are removed to create a buffed, generally smooth, treadless surface along the circumference of the tire casing to which a new layer of tread may be bonded.
The tire casing is then typically inspected for injuries, some of which may be skived and filled with a repair gum while others may be severe enough to warrant rejection of the tire casing. After completion of the skiving process, the buffed surface may be sprayed with a tire cement that provides a tacky surface for application of bonding material and new tread. Next, a layer of cushion gum may be applied to the back, i.e., the inside surface of a new layer of tread, or alternatively, the layer of cushion gum may be applied directly to the tacky surface on the tire casing. There are other known methods that may eliminate the need for cement or cushion gum. Conventionally, the cushion gum is a layer of uncured rubber material. The cushion gum and tread may be applied in combination about the circumference of the tire casing to create a retreaded tire assembly for curing. As an alternative, a length of tire tread may be wrapped around the tire casing with the cushion gum already applied. The cushion gum may form the bond between the tread and the tire casing during curing.
Conventionally, the buffing of the tire casing is controlled and manipulated by a human operator of a buffing machine. The buffing machine includes a rasp that can be applied to the surface of the tire to remove rubber. Because the circumference of a tire casing can more easily be measured by the operator, the desired final radius of the tire casing is typically identified in relation to the final circumference of the tire casing. It will be understood that circumferential measurements in this application are equivalent to radial/diameter measurements in that they are related by π. As the final desired circumference approaches, the operator may take a measuring device (e.g. a tape measure) and wrap it around the circumference of the casing to obtain a measurement and guess at the depth of each subsequent cut. This imprecise and error ridden process is time consuming and often results in a tire casing with a radius or circumference that does not substantially meet the final desired circumference.
One manner to reach the final circumference is to move the rasp into the desired position matching the final desired radius (or circumference) and keep repeating passes over the casing until there is no rubber being removed. This is very inefficient and unsatisfactory, and is thus not typically practiced.
There are a multitude of problems that may result from the imprecision of uncontrolled buffing of the tire as is typically practiced through the operator guessing the depth of final cuts or intentionally permitting imprecision. The treads on the tires are sometimes in the order of an inch or so in depth. On certain vehicles, two tires may be mounted next to each other on the same axle. If the radius of the tires is not substantially the same, a condition commonly referred to as scrubbing may occur. Such condition may derive from the radius of each final tire being different and, consequently, the velocity at the outer surface of the tire having a larger radius will tend to be larger than that of the tire having the smaller radius. The difference in velocity, given that the tires are mounted on the same axis, can generate friction in the tires at their outer surfaces, which tends to wear or scrub material from the tires in undesirable manners.
Another problem, among many others, generated by imprecision in arriving to the final circumference of a tire casing is that the tread to be applied may not match the casing, e.g., it may be too short or too long. In some instances, the tire tread is pre-cut to length before the tire casing is prepared while in other cases, the tread needs to be cut so that the tread pattern is continuous over the splice. In such instances, the predetermined length of the tire tread is expected to match the circumference of the buffed tire casing. When the circumference of the tire casing does not correlate properly with the predetermined length of the tread, which can occur through variability in the final outer circumference of the buffed tire casing, the tread will not match. In any event, the failure to substantially match final circumference of the tire casing to the target circumference of the tire casing and the predetermined length of the tread to be bonded to the tire casing can result in undesirable performance deficiencies.
A further undesirable problem with prior tire buffing has been that there can be delays and time inefficiencies as the operators attempt to experiment with the placement and operation of the rasp as rubber is being removed from the tire. Such time inefficiency and placement errors again result in undesirable performance deficiencies.
All of the noted problems and undesirable deficiencies are exacerbated further by the variability in the condition of incoming worn tires (or tire casings) that are commonly experienced. Incoming worn tires or tire casings often have rubber that is of differing characteristics depending on the environmental conditions to which they have been subjected in service. For example, tire casings that have been subjected to sustained heat might be more brittle than other tire casings that are younger and have not been subjected to heat. Some tire casings might have been stored for long periods in warehouses. Such and other historical circumstances of the incoming tire casings result in a significantly variable rubber product. Consequently, each buffing operation for different tires proceeds with different efficiencies and characteristics. Such differences further contribute to the final casing being variable and resulting in the noted problems.